Regulated vacuum-tube bias supply



June 4, 1957 w. G. WADEY REGULATED VACUUM-TUBE BIAS SUPPLY Ffiled June16, 1953 IN VEN TOR. WALTER 6. Wnos Y x SRQSQ (wink 770 EYS UnitedStates Patent REGULATED VACUUM-TUBE BIAS SUPPLY Walter G. Wadey, Hamden,Conn., assignor, by mesne assignments, to the United States of Americaas represented by the Secretary of the Navy Application June 16, 1953,Serial No. 362,056

1 Claim. (Cl. 250-27) The invention relates to voltage-regulatorcircuits and more particularly to a circuit for maintaining thegrid-tocathode potential of a vacuum tube or tubes substantiallyconstant, independent of the grid current flowing in such tube or tubes.

Grid bias for voltage amplifiers and power amplifiers is usually derivedfrom the plate-supply system. A common method of providing such bias isby using a resistor between the cathode and ground that makes thecathode positive by the resistance drop produced by the total spacecurrent in the tube. Another method of providing grid bias is by using agrid-leak (resistor) in the grid circuit of a vacuum tube, such as, aresistor inserted between'the grid and cathode of such tube. Stillanother method for providing grid bias is by inserting a bias cell orbattery in the grid circuit of a vacuum tube to make thegrid negativewith respect to the cathode.

" When grid leak and cathode resistors are employed to provide gridbias, there results variations in the potential developed across suchresistors, said variations being caused by the fluctuations in currentflowing through them. These variations produce undesirable variations inthe output of the amplifier tube. Bias cells or batteries providesubstantially constant grid-to-cathode potential, however, the cost ofutilizing such devices can become exorbitant.

It is an object of the present invention to eliminate theabove-mentioned disadvantages and provide a circuit for maintaining thegrid-to-cathode potential of a vacuum tube substantially constant,independent of the grid current flowing.

Another object of the invention is to provide a device for obtainingconstant bias potential from the potential drop across a tube.

The invention will best be understood from the following description ofa specific embodiment when read in conjunction with the accompanyingdrawings wherein:

Fig. l is a block diagram, partly schematic, of the invention includingthe regulator tube for maintaining the grid-to-cathode potential of avacuum tube substantially constant. Fig. 2 is a schematic wiring diagramof the invention of Fig. 1.

In the Figs. 1 and 2 there is shown the circuit for maintaining the biaspotential constant, said circuit consisting of a regulated power supply1 suitably connected to a two-stage amplifier 3, which in turn isconnected to a three-electrode tube 5. The grid element of saidthree-electrode regulator tube or grid leak 5 is used to regulate thebias potential of a vacuum tube or tubes, or the potential across adevice the resistance of which can be controlled electronically andcapable of drawing 1 current.

a resistance for limting the direct current to the desired value and twovoltage-stabilizing devices connected in series across the line formaintaining the voltage constant despite fluctuations in the voltage ofthe alternating current source or of variations in the load. Saidregulated supply provides power for operating the amplifier tube 7.

The two-stage amplifier circuit 3 includes a duotriode vacuum tubehaving two plates 9 and 11, control grids 13 and 15 and cathodes 17 and19 suitably connected to the power supply 1 and grid leak 5. Resistors27, 29, 30 and 31, in the plate, grid and cathode circuits respectivelyof tube 7, provide the proper voltages for operation of the amplifier.Said grid-leak tube 5 is an aircooled triode having a plate 21, grid 23and filament 25. The output voltage of regulator tube 5, taken acrossplate 21 which is grounded and cathode 25 is supplied to a vacuum tube6' as bias potential.

Tube 5 connected to amplifier circuit 3 acts as a gridleak resistor. Thecathode-to-plate resistance of tube 5 can be varied by varying thepotential applied to grid 23. Amplifier 7 amplifies deviations ofpotential drop across tube 5 and applies the resulting signal to thegrid of said tube 5 in such a sense so as to correct the deviations ofpotential. To accomplish this, plate 21 of tube 5 is connected to grid15 through resistors 33 and 30, thereby providing a negative feed-backloop and plate 9 of tube 7 is connected to grid 23 of tube 5 providing aforward loop for applying the amplified deviation to grid 23 ofgrid-leak tube 5. The net eflFect is to maintain the potential dropacross tube 5 substantially constant regardless of the current flowingthrough it. The current that flows through tube 5 is the grid current ofthe tube controlled. i

To illustrate further the operation of this device, assume that the biason a controlled tube 6 increases. The controlled tube can be a vacuumtube having a minimum of three electrodes, namely, a plate, cathode andcontrol grid. Such an increase could be caused by an increase in thegrid current drawn by the controlled tube. Because of this increase inbias there results a change of potential in wire or grid lead 35connecting plate 11 of tube 7 to filament 25 of tube 5 to a morenegative value. The potential differences between plate 11 and grid 13and between said plate and junction 37 are maintained constant byvoltage-regulator tubes in the regulated power supply 1. Thus, a changeof potential in grid lead 35 causes a change of potential of grid 15 toa value more positive with respect to its cathode 19 resulting inincreased conduction in that half of tube 7.

An increase of space current between cathode 19 and plate 11 causes anincrease in the potential drop across cathode resistor 31. Since thepotential of grid 13 with respect to junction 37 is maintained constantdue to power supply 1, the potential of cathode 17 will change to avalue more positive with respect to grid 13, or stated another way, grid13 becomes more negative with respect to cathode 17. This results in adecrease of space current between cathode 17 and plate 9 and a decreasedpotential drop across plate resistor 27. The potential of plate 9therefore changes to a value more positive with respect to grid lead 35.

Plate 9 of tube 7 is connected directly to grid 23 of tube 5 and is alsoconnected through resistor 27 to filament 25 of tube 5. The potentialchanges described above cause grid 23 of tube 5 to assume a potentialmore positive with respect to filament 25 thereby decreasing thepotential difference between said filament and plate 21. However, thispotential difference is the bias for the tube 6 to be controlled. Thus,the original increase in the bias of the controlled tube is compensatedfor by tube 5 producing a decrease in grid current to be fed to saidcontrolled tube.

The filament-to-plate potential of grid leak 5 is dependent upon thegrid current drawn by the controlled tube or tubes. As an example: Ifthe invention were to provide a bias supply for ten tubes, each of saidtubes drawing a grid current of 5 milliamperes, (ma), the total currentpassed by grid leak 5 would be of the order of 50 ma. In one embodiment,tube 5 is an RCA type 845. Its characteristics are such that with aplate current of 50 ma and a grid bias of minus 150 volts, theplate-tofilament potential difference is 1000 volts. This would be astable operating condition providing a regulated bias of 1000 volts forthe tubes to be controlled.

The bias potential of grid leak 5 is developed by the grid current ofthe controlled tube or tubes flowing from plate 21 to filament 25 oftube 5. When said tube 5 conducts, filament 25 will be negative withrespect to plate 21, the potential difference being 1000 volts. It willbe seen that any variation in the potential of grid 23 of tube 5produced by the action of the regulator circuit will vary thefilament-to-plate potential difference at a a constant plate current(the grid current of the controlled tubes).

The invention is not to be construed as limited to the particularregulator tube, duotriode amplifier circuit or load circuit as describedherein, since these are regarded to be illustrative rather thanrestrictive. For example, any equivalent device the resistance of whichcan be controlled electronically permitting utilization of the currentflowing through it can replace the load circuit described herein. Anytransistor amplifier circuit can be used so long as it amplifies thepotential deviations and applies them in the proper sense to theregulator tube.

What is claimed:

A voltage regulator circuit for producing a regulated bias potentialcomprising a two-stage amplifier circuit, a regulated power supplyconnected to the amplifier circuit for providing operating voltages tosaid circuit, a controlled vacuum tube having a plate, control grid andcathode, said tube drawing grid current, a grid-leak tube having aplate, control grid and filament, a capacitor connected across thefilament and plate of said gridleak tube, means for connecting thefilament of said grid-leak tube to the control grid of said controlledtube, whereby the grid current of the controlled tube flows throughsaid, grid-leak tube setting up a variable potential difference acrosssaid tube, means for applying deviations in potential appearing acrosssaid gridleak tube to one stage of said amplifier circuit, means forapplying the output of said first amplifier stage to the input of thesecond amplifier stage, and means for applying the amplified output ofsaid second stage to the grid of said grid-leak tube in such a sense soas to minimize the potential variations across the grid-leak tube,whereby constant bias potential across said grid-leak tube ismaintained.

References Cited in the file of this patent UNITED STATES PATENTS1,649,778 Hammond et al. Nov. 15, 1927 2,372,432 Keizer Mar. 27, 19452,394,891 Bowie Feb. 12, 1946 2,475,613 Hastings July 12, 1949 FOREIGNPATENTS 472,326 Great Britain Sept. 22, 1937 OTHER REFERENCES Theory ofIndustrial Electronics, Gage and Bashe,

35 McGraw-Hill, New York, 1951, Fig. 3-11, p. 50. (Copy in Patent OfficeLibrary.)

